Process for continuous modification of dihydrate gypsum and modified dihydrate gypsum obtained by the process

Patent metadata

Field	Value
Publication number	US-9458025-B2
Application number	US-201313962601-A
Country	US
Kind code	B2
Filing date	Aug 8, 2013
Priority date	Jul 31, 2008
Publication date	Oct 4, 2016
Grant date	Oct 4, 2016

Abstract

A process for the continuous modification of dihydrate gypsum includes calcining dihydrate gypsum into hemihydrate gypsum and recrystallizing the hemihydrate gypsum in an aqueous slurry to convert the hemihydrate gypsum into modified dihydrate gypsum of different crystalline form. The aqueous slurry in a recrystallization reaction tank is maintained at a constant temperature under stirring, and a feed rate of the hemihydrate gypsum to the recrystallization reaction tank and a discharge rate of the recrystallized dihydrate gypsum are controlled to substantially equal. Dihydrate gypsum as a starting raw material can be converted into high-purity, modified dihydrate gypsum of large crystals by recrystallizing the starting dihydrate gypsum after it is once calcined into hemihydrate gypsum.

First claim

The invention claimed is:

1. A process for modifying dihydrate gypsum, comprising:
a calcination step of subjecting the dihydrate gypsum as a starting raw material to calcination so as to convert the dihydrate gypsum into hemihydrate gypsum;
a recrystallization step of forming a first slurry comprising the hemihydrate gypsum, which has been obtained in the calcination step, contained as solid matter therein, hydrating the hemihydrate gypsum, and recrystallizing the hemihydrate gypsum in the first slurry in the presence of at least one material selected from the group consisting of a surfactant and a defoaming agent so as to modify the hemihydrate gypsum into recrystallized dihydrate gypsum having a higher brightness compared with the dihydrate gypsum as the starting raw material, thereby forming a second slurry; and
then an impurity separation step of removing the impurities comprising the darkness components, which provide darkness to the modified dihydrate gypsum, from the second slurry comprising the recrystallized dihydrate gypsum by a water-elutriation method, so as to form the modified dihydrate gypsum in a form of a third slurry,
wherein a solid content of the first slurry is in a range from 10 to 50 mass %, and
the water-elutriation method separates and eliminates the impurities from the second slurry by using a difference of a sedimentation rate between crystals in the modified dihydrate gypsum and the impurities, and discharging supernatant water containing the impurities.

2. The process according to claim 1 ,
wherein in the recrystallization step, the at least one material is incorporated in an amount range from 0.01 to 0.2 wt % based on the dihydrate gypsum as the raw material.

3. The process according to claim 1 , wherein the impurity separation step further comprises:
adding water to the recrystallized dihydrate gypsum separated from the darkness components by the elutriation method, and thereby forming the modified dihydrate gypsum into the third slurry; and
then dewatering and washing the modified dihydrate gypsum into a final product.

4. The process according to claim 1 ,
wherein the surfactant comprises at least one surfactant selected from the group consisting of alkylbenzene-sulfonate-based surfactants, alkyl-sulfate-based surfactants, alkyl-ether-sulfate-based surfactants, ligninsulfonate-based surfactants, naphthalene-sulfonate-based surfactants, bisphenol-based surfactants, and polycarboxylate-based surfactants.

5. The process according to claim 1 ,
wherein the defoaming agent comprises at least one defoaming agent selected from the group consisting of polyether-based defoaming agents, fatty-acid-ester-based defoaming agents, mineral-oil-based defoaming agents, silicone-based defoaming agents and emulsion-based defoaming agents.

6. The process according to claim 1 ,
wherein the calcination step, the recrystallization step and the impurity separation step are continuously conducted in this order.

7. The process according to claim 1 , wherein the process is a continuous process.

8. The process according to claim 1 , in which the hemihydrate gypsum is modified into the modified dihydrate gypsum continuously by supplying the hemihydrate gypsum continuously or intermittently to the recrystallization step, and separating the recrystallized dihydrate gypsum from the darkness components so as to form the modified dihydrate gypsum in the form of the third slurry continuously or intermittently.

9. The process according to claim 1 ,
wherein the first slurry used in the recrystallization step has pH in a range of 7.0±2.0.

10. The process according to claim 1 , further comprising:
removing water from the modified dihydrate gypsum in the third slurry so as to form dewatered modified dihydrate gypsum.

11. A process for modifying dihydrate gypsum, comprising:
a recrystallization step of forming a first slurry comprising hemihydrate gypsum contained as solid matter therein, hydrating the hemihydrate gypsum, and recrystallizing the hemihydrate gypsum in the first slurry in the presence of at least one material selected from the group consisting of a surfactant and a defoaming agent so as to modify the hemihydrate gypsum into recrystallized dihydrate gypsum thereby forming a second slurry; and
then an impurity separation step of removing the impurities comprising the darkness components, which provide darkness to the modified dihydrate gypsum, from the second slurry comprising the recrystallized dihydrate gypsum by a water-elutriation method, so as to form the modified dihydrate gypsum in a form of a third slurry,
wherein a solid content of the first slurry is in a range from 10 to 50 mass %, and
the water-elutriation method separates and eliminates the impurities from the second slurry by using a difference of a sedimentation rate between crystals in the modified dihydrate gypsum and the impurities, and discharging supernatant water containing the impurities.

12. The process according to claim 11 , further comprising:
removing water from the modified dihydrate gypsum in the third slurry so as to form dewatered modified dihydrate gypsum.

Assignees

• Yoshino Gypsum Co

Inventors

• Yokoyama Itaru
• Katsumoto Kouji
• Kawamura Yasuo

Classifications

• C01F11/466 Primary

  Conversion of one form of calcium sulfate to another · CPC title

• C01P2006/11

  Powder tap density · CPC title

• C01P2004/03

  obtained by SEM · CPC title

• C01P2004/61

  Micrometer sized, i.e. from 1-100 micrometer · CPC title

• C04B11/007

  After-treatment of the dehydration products, e.g. aging, stabilisation · CPC title